a1 Medical Research Council, Dunn Human Nutrition Unit, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, UK
a2 Medical Research Council, Human Nutrition Research, Downhams Lane, Milton Road, Cambridge CB4 1XJ, UK
Food samples (n 114) were prepared from vegetables commonly eaten in Europe. The glycosidic forms of the phyto-oestrogens daidzein and genistein were extracted from the dried foods into aqueous methanol. The isoflavones were quantified by GC–MS after hydrolytic removal of any conjugated carbohydrate. Completeness of extraction and any procedural losses of the isoflavones were accounted for using synthetic daidzin (7-O-glucosyl-4′-hydroxyisoflavone) and genistin (7-O-glucosyl-4′5-dihydroxyisoflavone) as internal standards. Of the 114 foods assayed, at a limit of quantification of 0·1 μg/kg dry weight, forty-eight contained no detectable daidzein or genistein, forty-one contained less than 100 μg/kg of the two isoflavones combined and the remaining twenty-five contained more than this amount. Soyabean products contained between 470 and 1420 mg (average of 960 mg) daidzein and genistein combined per kg wet weight of food, and legumes contained between 20 and 5750 μg/kg wet weight of food, with an average of 620 μg/kg. Cooking by boiling in water caused a decrease in the daidzein and genistein content of food in twenty-four of twenty-eight foods. The extent of the decrease was variable and warrants further investigation. The present paper comprises the first comprehensive description of the content of daidzein and genistein in vegetables.
(Received November 11 1999)
(Accepted April 19 2000)